Acoustic filter with two different channels with compensation for rejection

Abstract

A surface acoustic wave filter includes first and second channels, each channel including at least one input transducer and one input transducer. The transfer functions of the first channel and of the second channel are in phase in the passband of the filter and in antiphase in the rejection band of the filter. The filter enables the use of frequency responses for each channel, with wide bands enabling high reduction of the size of the filter while at the same time reducing the rejection band of the total filter. Such a filter may find particular application to mobile telephony.

What is claimed is:  
     
       1. A surface acoustic wave filter comprising transducers, each transducer being defined by a weighting function representing a distribution of centers of reflection and centers of transduction of said transducer, said filter having a passband and a rejection band and comprising: 
       a first channel comprising at least one first input transducer and one first output transducer defining a first transfer function, said first channel having a first resultant weighting function;  
       a second channel comprising at least one second input transducer and one second output transducer defining a second transfer function, said second channel having a second resultant weighting function;  
       wherein the first and second input transducers are electrically connected to each other;  
       wherein the first and second output transducers are electrically connected to each other;  
       wherein the resultant weighting functions of the first and second channels are different and such that transfer functions of the first channel and of the second channel are in phase in the passband of the filter and in antiphase in the rejection band of the filter; and  
       wherein at least one of the first and second input and output transducers are series connected.  
     
     
       2. A surface acoustic filter according to claim  1 , wherein the first and second input transducers or the first and second output transducers have the same weighting function. 
     
     
       3. A surface acoustic filter according to claim  2 , wherein the first and second channels are indistinguishable from each other at input or at output to realize the same weighting function. 
     
     
       4. A surface acoustic filter according to claim  1 , wherein at least one of the first and second input and output transducers comprise a first series of electrodes connected to an external bus of a first differential or non-differential input, a second series of electrodes connected to an external bus of a second differential or non-differential input, a third series of electrodes connected to a central bus with a floating potential, said third series separating said first and second series of electrodes. 
     
     
       5. A method for making a filter according to any one of claims  1  to  4 , further comprising: 
       determining the resultant weighting functions of the transducers of the first channel and of the transducers of the second channel by an iterative process so as to define transfer functions of said first channel and of said second channel in phase in the passband of the filter and in antiphase in the rejection band; and  
       discretization the weighting functions to achieve electrode masks constituting the transducers of the first and second channels.  
     
     
       6. A surface acoustic wave filter comprising transducers, each transducer being defined by a weighting function representing a distribution of centers of reflection and centers of transduction of said transducer, said filter having a passband and a rejection band and comprising: 
       a first channel comprising at least one first input transducer and one first output transducer defining a first transfer function, said first channel having a first resultant weighting function;  
       a second channel comprising at least one second input transducer and one second output transducer defining a second transfer function, said second channel having a second resultant weighting function;  
       wherein the first and second input transducers are electrically connected to each other;  
       wherein the first and second output transducers are electrically connected to each other;  
       wherein the resultant weighting functions of the first and second channels are different and such that transfer functions of the first channel and of the second channel are in phase in the passband of the filter and in antiphase in the rejection band of the filter; and  
       wherein central frequencies of the first and second transfer functions are substantially equal.